Europium activated yttrium, gadolinium (Y,Gd)BO.sub.3 :Eu.sup.3+ is an efficient red emitting phosphor. Efforts have been made to improve the performance of this phosphor currently used in Plasma Display Panels (PDP) due to its high quantum efficiency, persistence characteristics and reduced saturation.
Eu.sup.3+ activated rare earth and alkaline earth borate phosphor is one phosphor candidate that has been investigated by Applicant herein, as described in copending U.S. patent application, Ser. No. 09/12,169, now U.S. Pat. No. 6,042,347, the disclosure of which is incorporated herein by reference. The aforesaid phosphor exhibits three narrow peaks in the red region.
These phosphors are typically prepared by a high temperature (&gt;1200.degree. C.) solid state reaction (SSR) between Y.sub.2 O.sub.3 (Y source), Gd.sub.2 O.sub.3 (Gd source), Eu.sub.2 O.sub.3 (Eu source), boric acid and a flux (NH.sub.4 F or NH.sub.4 Cl). The grain size of the phosphor powders prepared from SSR is of the order 5 to 10 microns. Flat panel display devices, such as PDP's, Field Emission Displays (FED) and Electro-Luminescence (EL) panels require thin fluorescent screens with fine grain (0.1 to 2.0 microns) phosphors for optimum performance and high efficiency. This requirement is more demanding in the case of PDP's, as the phosphors are screen-printed between the ribs to form a complicated structure. With small particles, it is possible to form a thin screen. Small particles also allow for a higher packing density and require less binder content to form an adherent thick film to its substrate.
Traditionally, small phosphor particles have been obtained by grinding, crushing or milling of larger phosphors into small particles. Phosphors obtained by these methods show greatly reduced efficiency with little or no control over the particle morphology. More recently, "no mill" phosphors have been prepared by rapid cooling of the mass after completing the SSR, and with either a short-time firing at a higher temperature, or a longer duration firing at a lower temperature. These processes help minimize further growth of phosphor crystals. In the presence of flux or inhibitors, particle size distribution (PSD) and morphology of the phosphor can be controlled. It has been proposed that sub-micron size phosphor particles can be synthesized by sol-gel methods.
Most prior art red phosphors are Eu.sup.3+ activated yttrium oxide for fluorescent lamps and yttrium oxy-sulfide for CRT's. Since these phosphors are not suitable for AC type PDP's, efforts are being made to develop new phosphors, which are excitable with 147 and 173 nm from a Xenon source in a PDP. Willi Lehmann (U.S. Pat. No. 4,202,794) proposed an improved phosphor composition expressed by the general formulation xCaO.y(Y+Eu).sub.2.zB.sub.2 O.sub.3 wherein x is from 32 to 38, y is from 31 to 40 and z is from 25 to 31, with x, y, and z expressing relative molar proportions of the constituents. The Lehman phosphor composition has a higher photoluminescence efficiency than similar type phosphors, and when excited by 254 nm radiation emits in the red region of the visible spectrum.
Chung-Nin Chau in U.S. Pat. No. 5,776,368 teaches an improved method for a single firing synthesis of a borate phosphor having a general formula (Y.sub.1-x-y Gd.sub.x Eu.sub.y) BO.sub.3, where x is from about 0.1 to about 0.3 and y is from about 0.05 to about 0.12. In addition to boric acid, boron nitride was used as a source of boron. The calcination was carried out at a solid state reaction temperature (1250.degree. C. for 3 hours).
Japanese patent 59-15951 to Tsujimoto et al. describes a rare earth borate based phosphor composition LnBO.sub.3 :Eu,Bi (where Ln is Y or Gd) for plasma display applications These phosphors are synthesized by firing respective oxides at 1250.degree. C. for 3 hours. Huguenin et al. (WO 97/26312) describe a method of making Eu.sup.3+ activated (Y,Gd) BO.sub.3 phosphors by calcinating rare earth carbonates as well as rare earth hydroxycarbonates with boric acid at 1100.degree. C. for 10 hours.
Solid state reactions, involve high temperature calcination (&gt;1100.degree. C.). With this high temperature process, the control over impurity concentration, distribution of activators in the bulk, single phase formation, PSD and morphology are limited. It has been found that the phosphor screens formed with small particles (0.5 to 2.0 microns) exhibit improved performance. This is particularly true for PDP's. The growth of small particles through use of sol-gel methods, where the reaction temperatures are well below the normal solid state temperatures (&lt;1000.degree. C.).